1. Field of the Invention
This invention relates generally to the field of seismic gas exploders adapted to rest on the earth and deliver multiple compressional pulses into the earth by operation in a cyclic fashion. The invention is more particularly concerned with exploders of this character which are operated at a high firing rate over a long sequence of shots.
2. Description of the Prior Art
Seismic gas exploders or as they are sometimes termed, seismic wave generators, broadly speaking include a detonation of combustion chamber having a rigid bottom and a rigid top telescopically joined in some fashion to permit relative vertical movement therebetween. Initiation of the explosion of a fuel gas and oxidizing gas combination within the combustion chamber produces the desired relative movement and applies a compressional pulse to the earth through the rigid bottom of the chamber.
Normally, fuel gas and oxygen from separate containers are mixed prior to being conducted into the detonation chamber in correct proportions by means of flow controlling solenoid valves interposed between the separate containers and the mixing point. Upon closing of the solenoid valves, the combustible gas mixture is ignited by a spark source located at the point of mixing. The resulting combustion accelerates into a detonation front within the inlet pipe or conduit leading to the exploder's detonation chamber.
The effects of heat generated by the chemical reaction involved in the burning and detonation of such a gas mixture do not of course dissipate immediately. If a gas exploder is fired repetitively at a rapid rate, one of the barrier problems is ignition of gases during fill prior to activation of the spark source, i.e., preignition. The higher the rate of fire, the sooner this limitation is manifested. In practicing modern seismic data acquisition techniques, it is desirable to be able to generate long pulse sequences, ranging, for example, up to 100 to 200 pulses. For this and other reasons, therefore, the problem of preignition has assumed greater importance.
Premature ignition may be the result of several factors including but not necessarily limited to: (1) residual heat stored in the walls of the exploder, inlet conduit or mixer-spark ignition assembly; (2) residual flame due to combustion chamber of gases not burned in the detonation front (to be distinguished from failure of solenoid valves to close completely, resulting in a sustained flame); (3) heat released by carbon in gas form converting to solid form; (4) glowing particles adhering to walls of the exploder and inlet, and (5) hot spots at localized points due to detonation and subsequent shock waves.
The prior art has recognized the existence of the preignition problem; but so far as Applicant is aware, it has assumed that among the factors mentioned above the main contribution has been the buildup of heat within the detonation chamber itself. Thus, for example, exhaust valving arrangements have been devised more efficiently to release the spent gases from the explosion. This approach has met with little success. Applicant has by contrast focused his attention outside the detonation chamber.
In approaching the preignition problem in a repetitively fired gas exploder, Applicant first considered that significant advantage might be achieved by reducing the frequency with which an individual gas inlet and its associated external conduit and mixer assembly were employed without reducing the firing rate. In copending application Ser. No. 751,407, entitled METHOD AND APPARATUS FOR GENERATING A LONG SEQUENCE OF SEISMIC PULSES filed concurrently herewith in the name of Tom P. Airhart, a method and apparatus is described which teaches the reasonableness of this assumption.
Applicant has continued to search for still further improvement in the charging and ignition systems utilized with gas exploders of the character described. The present application therefore deals with such further improvement.